1. Field of the Invention
The present invention relates to an organic electroluminescent element capable of emitting light by converting electric energy into light (hereinafter also called organic EL element, luminescent element, or EL element).
2. Description of the Related Art
Various display elements are intensively researched and developed recently, and an organic electroluminescent (EL) element is noticed as a hopeful display element among others because light can be emitted at high luminance at low voltage.
The organic electroluminescent element is composed of counter electrodes having a luminescent layer or a plurality of organic layers including the luminescent layer therebetween, and makes use of luminescence from an exciton generated by rebonding of an electron injected from a cathode and a hole injected from an anode in the luminescent layer, or makes use of luminescence from an exciton of other molecule generated by energy transfer from the generated exciton.
It has been proposed to reduce power consumption or enhance driving durability by containing a hole trap compound in an electron transport layer (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 2004-79413, 2004-79414, and 2002-313583).
It has been also proposed to enhance driving durability and low voltage driving by a method of forming a hole blocking layer containing plural types of electron transport materials between a luminescent layer and an electron transport layer, or by a method of containing an aromatic diamine compound in an electron transport layer (see, for example, JP-A Nos. 2001-237079 and 5-311161).
However, organic electroluminescent elements using phosphorescent materials as luminescent materials (phosphorescent elements) are low in durability particularly in blue and green luminescence, and improvement of durability thereof is demanded.